Currently in the United States a significant portion of new motor vehicles are front-wheel drive based Vehicles. In a front wheel drive vehicle, both the engine and the transmission assembly are typically transversely oriented in the vehicle. A transaxle combines the functionality of the transmission, the differential and the drive axle into one integrated assembly.
Front wheel drive vehicles provide advantages over traditional rear wheel drive vehicles. In smaller front wheel drive vehicles, the engine weight above the front wheels, the drive wheels, provides for increased traction over the relatively lighter rear end of the vehicle. Thus, when a vehicle is driven on slippery surfaces such as a wet roadway or a roadway with a loose surface such as dirt or gravel, the majority of weight is located over the driving (front) wheels. The additional weight of the engine increases the traction between the front wheels and the surface of the road. Increased traction aids the vehicle in turning and reduces the potential for losing traction and skidding or sliding on the road surface. As such, it is advantageous to equip a vehicle with front wheel drive when the overall weight of the vehicle is relatively low.
Recently the automobile industry has experienced a trend in which consumers are purchasing all-wheel drive or four wheel drive vehicles. Consumers have displayed an affinity for larger, heavier, and roomier vehicles that blend the rugged capabilities of a truck with the comfort of a passenger vehicle. These hybrid vehicles have become known as sport utility vehicles or “SUVs.” The popularity of SUVs has increased dramatically over the last decade, and the drive systems of these vehicles have become more and more sophisticated. Usually these SUVs are equipped as rear wheel drive vehicles with a selectable four wheel drive mode.
The popularity of four wheel drive SUVs has led the automotive industry to offer conventional automobiles with four wheel drive or all wheel drive options. Thus, smaller vehicles are being produced with four wheel drive modes. The drawback to this is that four wheel drive vehicles typically have lower fuel economy than standard two wheel drive vehicles. With the recent increases in oil prices, fuel economy continues to be a concern for drivers and the need exists for a four wheel drive system that is fuel efficient while still providing the increased traction of four wheel drive.
In addition to consumers becoming increasingly concerned with fuel economy, many consumers are enjoying the extra benefit of being able to drive the vehicle on trails and paths where the extra traction provided by four wheel drive is required to avoid becoming stuck. This activity, commonly known as “off-roading,” or “four-wheeling,” has gained in popularity. Indeed, many groups now exist solely for the purpose of driving vehicles off-road in groups. In fact, many clubs and organizations now exist that are devoted to driving vehicles off road. Additionally, various racing events now use four wheel drive vehicles in off road races.
As drivers have continued to drive off road and the opportunities for off-road driving have increased, a need has arisen for gearing in vehicles to facilitate off-road driving. Driving a vehicle off a paved road can sometimes involve very slippery or steep conditions. In steep driving conditions, there is a need for the driver to maintain the vehicle at a slow pace. Vehicle speeds normally associated with driving on paved roads typically can be too fast for off-road conditions. While vehicles are equipped with brakes that will slow the vehicle, excessive use of the brakes can overheat the brake pads leading to a decrease of brake efficiency. Thus, gearing inside the transmission is often used to maintain a slower speed when a vehicle is off-road. Typically, an off-road vehicle will have a second set of gears for use off-road. This second set of gears usually has higher torque output and slower speeds than the gears used for normal operation on paved roads. A driver will disengage the normal driving gears and engage the off-road gears (typically called “low-range” gears) once the vehicle leaves the pavement. This second set of gears provides a wider range of torque output and higher traction for a four wheel drive vehicle, thus providing a driver with greater control of power output to the wheels and aiding in operating the vehicle at a slow speed.
As a result of off-road driving, it is entirely possible and, in fact, somewhat likely in certain driving conditions, that a vehicle will encounter a spot where it loses traction and becomes unable to move; it gets stuck. Once a vehicle becomes stuck, various rescue techniques can be used to free the vehicle from the area in which it is stuck. One of the most effective ways to free a stuck vehicle is to tow it with another vehicle. For effective towing, a vehicle should have gears that allow the transmission/transaxle to enter into a neutral range, thus disengaging the drive train from the transaxle and freeing all four wheels to turn independently of the vehicle's engine. Thus, the automobile industry has incorporated neutral ranges into the transmissions and transaxles of four wheel drive and all wheel drive vehicles. The neutral range provides for a simple, effective method to place a vehicle in proper condition to be towed.
The automotive industry has responded to the desire to combine the benefits of four wheel drive, alternative gearing resulting in increased traction, additional torque ranges, and a neutral range, with a fuel efficient engine. Thus, there has been a trend to adapt front-wheel drive schemes and the advantages of front-wheel drive, with the advantages of all-wheel or four-wheel drive applications. A particular challenge has been to also provide additional torque ranges within known front-wheel drive transaxle configurations that can produce torque multiplication and lower vehicle operating speeds. Further, it has been advantageous to provide a neutral range that will allow decoupling of the all-wheel or four-wheel system from the transmission, thus allowing for easy towing.
A variety of power transfer units for front wheel drive vehicles have been described previously and are known in the prior art. None of the power transfer units known in the art, however, are designed to solve the particular problems addressed by the present invention. Therefore, a need still exists for a front wheel drive, transverse based drivetrain with a simple, mechanical clutch and a planetary gear set that allows a greater range of gear ratios including low range, for off road driving, a high range for normal operation of the vehicle, and neutral for use in towing the vehicle.